1. Field of the Invention
This invention relates to an apparatus for cleaving optical fiber waveguides; and more particularly, to an automatic apparatus for cleaving an optical fiber waveguide held in tension to produce a planar end surface perpendicular to the fiber axis and suitable for fusion splicing.
2. Description of the Prior Art
Transmission of data by optical fiber waveguides, also called fiber optics or optical fibers, has become ubiquitous in the telecommunications and computer industries. Digital information in an electronic system is converted into a series of pulses of light generated by lasers or light emitting diodes (LED's), which are injected into long fibers of glass or polymeric materials. The fibers are capable of propagating the light with extremely low losses and acceptably low dispersion, whereby information embodied in the modulation pattern may be conveyed. The light that emerges from the other end of the fiber can be detected and reconverted into electronic signals that faithfully reproduce the original signal.
Fiber optic communication has a number of advantages over traditional transmission means such as hard-wired coaxial and twisted pair cable and lower frequency electromagnetic broadcasting such as radio and microwave. Foremost is the much larger bandwidth available. In addition, existing infrastructure such as cable ducts, utility poles, and the like presently used by telecommunications companies can be upgraded with relatively little disruption and moderate cost by substituting optical fiber cable for existing copper wire. Thus, dramatic increases in bandwidth needed to accommodate the needs of an information-based, Internet-driven society and commerce can be obtained with comparatively little disruption.
The bandwidth of a given optical communications system is further increased by the use of polarization-maintaining (PM), single mode optical fiber. Such PM fiber is characterized by some form of azimuthal asymmetry that results in very different propagation constant modes of two orthogonal polarizations. Cross-coupling of the modes is very low, typically at a level of −20 to −30 dB.
Implementation of fiber optic systems requires both the equipment for actual transmission and processing of the data, and the equipment needed to install and maintain the fiber optic system and its infrastructure. The transmission and processing equipment, such as the fiber itself and the corresponding components needed to generate, detect, and process optically-borne information, have been developed to an ever increasing level of sophistication. While certain systems for joining and splicing fiber optic cables have been developed, there remains a need in the art for improved equipment and methods for splicing that are reliable, economical, and which result in minimal loss of signal integrity and strength, especially for the polarization-maintaining fibers. Such systems, equipment, and methods are essential if the full inherent advantages of optical transmission are to be more widely implemented.
Together, these considerations call for splicing systems that are compact, portable, and able to be operated rapidly and reliably under adverse working conditions and with minimal slack cable. Moreover, it is desired that such a splicing system be capable of joining two fibers in a way that (i) causes minimal disruption discontinuity in the optical transmission, (ii) does not adversely increase the diameter and volume of the cable, and (iii) has a durability as close as possible to that of an original fiber. Systems are also desired that are simple and reliable enough to be used by technicians who lack extensive training. There remains an urgent need for optical splicing equipment satisfying these requirements.
Optical fiber waveguides in common use share a number of structural features. The waveguide almost invariably comprises a thin, elongated fiber core responsible for conducting the light and at least one additional layer. Most often the fiber core is highly pure glass surrounded by a first and intimately-bonded layer termed a cladding and an outer layer called a buffer. The cladding, usually also glass, has an index of refraction lower than that of the core to insure that light is constrained for transmission within the core by total internal reflection. Typically, the buffer is composed of plastic or polymer and serves to protect the inner layers mechanically and to prevent attack by moisture or other substances present in the fiber's environment. Commonly a plurality of individual fibers (in some cases as many as a thousand) constructed in this fashion is bundled together and enclosed in a protective jacket to form a cable.
Commonly used fibers may further be classified as multimode or single mode. Multimode fibers typically comprise cores having diameters of 50–62.5 μm but in some cases up to 100 μm. Single mode fibers generally have a much smaller core that may be 9 μm or less in diameter. The glass-cladding diameter is most commonly 125 μm but sometimes is 140 μm (with a 100 μm core). The exterior diameter is largely a function of the buffer coating, with 250 μm most common, although some fiber coatings may be as much as 900 μm in diameter.
Two general approaches for splicing optical fibers are in widespread use, viz. mechanical and fusion splicing. Mechanical splicing is accomplished by securing the ends of two fibers in intimate proximity with an aligning and holding structure. Often the fibers are inserted into the opposing ends of a precision ferrule, capillary tube, or comparable alignment structure. The fibers are then secured mechanically by crimping, clamping, or similar fastening. An adhesive is also commonly used. In some cases a transparent material such as a gel having an index of refraction similar to that of the fiber cores is used to bridge the gap between the fibers to minimize reflection losses associated with the splice. Mechanical splicing is conceptually simple, and minimal apparatus is required to effect splicing. However, even in the best case, a mechanical splice has relatively high and undesirable insertion loss, typically 0.20 dB. In addition, mechanical splices are generally vulnerable to degradation of the optical quality of the splice over time, especially under adverse environmental conditions such as varying temperatures and high humidity. Mechanical splices are generally regarded as being temporarily expedients at best and are not useful for high bandwidth systems or permanent joints.
Fusion splicing entails the welding of the two fiber ends to each other. That is, the ends are softened and brought into intimate contact. The softening is typically induced by a small electric arc struck between miniature pointed electrodes mounted in opposition and substantially perpendicular to the common axis of the fibers. Upon cooling, a strong, low-loss joint is formed. When properly carried out, fusion splices exhibit very low losses along with high stability and durability rivaling those of the uncut fiber.
Careful preparation of the ends of the fibers being joined is essential for forming low loss splices in both ordinary and polarization maintaining optical fibers. In particular, each fiber end ideally should be a planar surface perpendicular to the fiber axis. However it is frequently found that existing methods produce surfaces having a variety of defects compromising this ideal surface. The defects include non-perpendicular surfaces, non-planar depressions or protruding asperities, and chips off the periphery of the terminal end. Each of these defects adversely impacts the quality of a fusion splice. It is also known that the brittle fracture of the glass material of the fiber requires careful control of the mechanism used to induce the fracture.
A number of devices, both manual and automatic, have been proposed for carrying out the cleaving operation. In generally, they rely on using a knife blade or similar hard surfaced tool to induce a surface mechanical defect on the circumference of the fiber and imparting a mechanical force to cause a crack to propagate from the induced defect to sever an expendable portion and leave a mating face.
However, existing devices generally lack the degree of control and automation needed for reliably producing a planar, perpendicular mating face for proper fusion splicing. They are relatively complex in design, construction, and operation. The consistency of results is highly dependent on the skill and attention of the operator. As a result, there is a significant need for a system wherein the cleaving process is carried out consistently to eliminate the unpredictable effects of operator action.